1. Field of the Invention
The present invention pertains to the field of digital communications. More particularly, this invention relates to switching hubs for high speed local area networks.
2. Background
A typical prior local area network is comprised of a set of communication agents coupled to a common communication medium. For example, an Ethernet local area network comprises a set of Ethernet compatible communication agents coupled to a coax cable or a twisted pair cable as a communication medium. In such systems a communication transaction between any two of the communication agents typically prevents communication among the other communication agents during the communication transaction.
One prior method for increasing the capacity of a local area network is to employ bridge circuits. Prior bridge circuits typically couple together segments of local area networks. Typically, each segment comprises a set of communication agents coupled to a communication medium. A bridge circuit enables communication between each of the communication mediums.
Such prior bridge circuits typically listen for communication messages on each of the communication mediums and propagates the appropriate communication messages to the other communication medium according to the network configuration. Communication messages are commonly referred to as communication frames.
Such a prior bridge circuit typically buffers communication frames that require propagation to another communication medium. Thereafter, the bridge circuit forwards the buffered communication frames to the appropriate destination communication agents. Unfortunately, such buffering and forwarding of communication frames between the communication mediums of the local area network increases the overall latency in the communication network.
Another prior method of increasing the capacity of a local area network is to employ a switched hub circuit. A switched hub circuit is typically arranged in a "star" configuration wherein each communication agent in a segment of the local area network couples to a unique serial communication port of the switched hub circuit. The switched hub circuit typically senses incoming communication frames, determines the destinations of the incoming frames, and switches the incoming communication frames to the appropriate outbound transmission paths.
Such switched hub circuits are typically faster than bridge circuits because switched hub circuits do not store and forward the incoming communication frame. Switched hub circuits instead usually start switching the incoming communication frames to the appropriate destinations immediately after determining the destinations of the incoming communication frames according the headers of the incoming communication frames. In addition, such switched hub circuits usually allow parallel data transfer between ports as long as the data transfer paths do not conflict.
Unfortunately, the switching functions performed by such prior switched hub circuits are complicated by certain types of communication traffic. For example, if more than one communication agent attempts to transmit to the same communication agent, the switched hub circuit must select one of the transmissions and delay the other transmissions. The delayed transmissions increase the latency in the network.
In addition, broadcast transmissions and multicast transmissions require a switched hub circuit to switch a single incoming communication frame to multiple destination communication agents even though conflicting traffic may be under way to the destination communication agents. The multicast transmissions typically conflict with other transmissions, thereby increasing delays in the network.